Method and apparatus for increasing the readability of amplitude modulated waves



w. R. LEWIS ETAL. 3,311,833 METHOD AND APPARATUS FOR INCREASING THEREADABILITY March 28, 1967 OF AMPLITUDE MODULATED WAVES Filed April ll,1963 United States Patent O 3,311,833 MIETHD AND APPARATUS FUR INCREASNGTHE READABILITY OF AMPLHTUDE MDU- LATED WAVES William R. Lewis, KansasCity, and William E. Hanneman, Independence, Mo., and George W. Wester,Mission, Kans., assignors to Wilcox Electric Company, lne., Kansas City,Mo., a corporation of Kansas Filed Apr. 1l, 1963, Ser. No. 272,437 4Claims. (Cl. 325-474) This invention relates to a method and apparatusfor increasing the readability of amplitude modulated, double side bandsignals.

It has been noted heretofore that the human ear is capable ofdistinguishing certain sounds or intelligence from background noise orundesired intelligence due to the differences in the phase relationshipsbetween the wanted and the unwanted sounds, For example, conversationemanating from a speaker directly in front of the listener reaches eachear of the listener at the same instant. Therefore, the sounds receivedby the listener are in phase with one another. Conversely, the soundsemitted by a speaker positioned to one side of the listener will notreach ear of the listener at the same time; hence, the sounds will reachthe listeners ears out-of-phase.

The human mind apparently tends to X its attention on sounds having acertain phase difference independent of the frequency thereof. Thus, themind is able to focus its attention on a particular sound source due tothe fixed phase difference between the sound waves reaching the earsfrom that particular source. Furthermore, the mind appears to place thesource of a sound best when the amplitudes at the ears are equivalent,or nearly so.

The above effect is, of course, helpful to one when it is desired toplace the source of a sound. The human ears might, therefore, bevisualized as a binaural or dual channel sound-receiving system which iscapable of placing the source of `a sound due to the phase differenceexisting between the two channels.

Although the effects discussed above are useful in determining thesource of an external sound, another eiect heretofore noted alsoenhances the minds ability to focus on a given sound. It has been foundthat when one sound wave is directed to one ear and another sound wave,carrying the same intelligence as the first wave, is directed to theother ear 180 degrees out-of-phase with the first wave, the source ofthe sound then appears to be located within the head of the listener,Therefore, other sounds not 180 degrees out-of-phase will appear to belocated externally of the listener and thus, attention may readily befocused on the intelligence received by the ears 18() degreesout-of-phase.

Various schemes have been proposed for utilizing this effect. One suchscheme is that of increasing the amount of useful information that canbe transmitted on a given number of channels by sending each message ona pair of channels. In this manner, several messages may be placed oneach channel, but the listener will be able to pick out thedesired'message by reversing the phase of the appropriate channel withrespect to a second appropriatc channel. Additionally, it has beenproposed that intelligence could be transmitted on two separate radiofrequencies along with random conversations or noise. Such randomconversations or noise, however, would 3,3 l f Patented Mar. 28, 1967ICC contain no like counterpart on the other channel, .e., only thedesired intelligence would he transmitted on both channels. Therefore,it is evident that the desired intelligence may be extracted by thelistener by the phase reversal technique.

Unlike the prior means of utilizing this effect, the present inventionis directed to a system for enhancing the reception of amplitudemodulated, double side band signals produced by conventional means andtransmitted to the listener. Generally, such signals during transmissionto the listener will take the form of radio waves, but the concepts ofthis invention are equally applicable to wire transmission systems. Theinvention may be utilized to increase the readability of normalamplitude modulated signals having an unsuppressed carrier and an upperand a lower side band, double side band partially suppressed carriersignals, or double side band fully suppressed carrier signals.

The means provided hy this invention for extracting the intelligencefrom amplitude modulated signals utilizes the listeners ability to focusattention upon the apparent origin of the desired intelligence when thatintelligence seems to origin-ate from a source which appears to bepositioned differently than the apparent source of the undesiredinterference. The desired intelligence in the side bands constitutes themajority of the audio components which retains one specic phaserelationship. Undcsired side bands (whose carriers are removed by a fewcycles or more), heterodyning carriers, random noise, intentionaljamming, et-c, either occur only in one of the reception channels of theinvention or where they appear in both channels, have random frequencyand phase relationships and, therefore, seem to originate from a randomspherical source about the listener. These unwanted signals give thesensation to the listener of moving; unwanted signals rotate around thehead of the listener in a manner characteristic of the particularchanging phase difference.

In brief, this is achieved in the instant invention by detecting eachside band separately and then feeding each of the detected signals todifferent ears of the listener, the phase of one signal being reversedwith respect to the other signal. By this scheme it is possible to readintelligence which otherwise would be completely masked by interferenceand unreadable when received by an amplitude modulated receiverofequivalent band width employing conventional techniques. Furthermore, itis noteworthy that some enhancement of the intelligence is achievedwithout reversing the phase of one of the detected side band signals,but by merely directing the intelligence in one side band to one ear ofthe listener while the intelligence in the other side band is directedto the other ear of the listener.

It is, therefore, the primary object of this invention to provide amethod and apparatus for increasing the readability of amplitudemodulated, double side band signals by detecting each side bandseparately to thereby obtain a pair of intelligence signals and thenreversing the phase of one of said intelligence signals,

It is another object of this invention to provide a method and apparatusfor increasing the readability of amplitude modulated, double side bandsignals by detecting each side band separately and directing each of thetwo channels of intelligence so obtained to different ears of thelistener.

It is another object of this invention to provide a method and apparatusfor increasing the readability of amplitude modulated, double side bandsignals by detecting each side band separately to obtain a pair ofintelligence signals, reversing the phase of one of said intelligencesignals, and then directing the phase-reversed intelligence signal toone ear of the listener while the other intelligence signal is directedto the other ear of the listener.

It is another object of this invention to provide apparatus forseparating the components of an amplitude modulated, double side bandsignal to permit separate detection of the side bands and subsequentphase reversal of the intelligence obtained from one of the side bands.

It is another object of this invention to provide apparatus forseparating the side band signal components from an amplitude modulated,double side band signal and for providing a de-modulating electricalsignal phase locked with the carrier signal component of the AM signalto permit separate detection of the side bands.

It is still another object of this invention to provide apparatus forgenerating a de-modulating carrier signal -for use in the detection ofamplitude modulated, double side band, fully suppressed carrier signalswherein said de-modulating carrier is of a frequency and phase such thatthe detected side bands will be in phase, and for converting thedetected side bands into a pair of sound waves 180 degrees out-of-phasewith one another.

It is yet another object of this invention to provide a scheme fordetecting amplitude modulated signals having a carrier signal componentand a pair of side band components wherein the components are eachseparated from the parent signal by filter means, the carrier cornponentbeing injected into the feedback circuit of a crystal oscillatora forphase locking said oscillator with the carrier component, the side bandcomponents being fed to individual mixers or product detectors alongwith the output signal from said oscillator to thereby effect detectionof each side band separately and permit subsequent phase inversion ofthe intelligence obtained from one of the side bands.

Other objects will become description proceeds.

In the drawing:

FIGURE 1 is a block diagram showing the basic system concept of thepresent invention applicable to arnplitude modulated, double side bandsignals having either an unsuppressed or a partially suppressed carrier;

FIG. 2 is a block diagram showing an improved detecapparent as thedetailed tion scheme for utilization with amplitude modulated,

double side band signals having a carrier signal component; and

FIG. 3 is a block diagram showing a modification of FIG. 2 for use withamplitude modulated, double side band, fully suppressed carrier signals.

Referring to FIG. 1, it may be seen that the antenna l0, radio frequencyamplifier 12, converter 14 and intermediate frequency amplifier 16comprise apparatus found in conventional superheterodyne receiversystems. At the output of the IF amplifier 16, the carrier signalcomponent of the incoming radio wave will be at the intermediatefrequency regardless of the actual frequency of the wave as it istransmitted. This, of course, is in accordance with well knownprinciples of superheterodyne receivers.

An upper side band filter 1S, a lower side band filter 20, and a carrierfilter 22 are each coupled with the output of IF amplifier 16. The upperand lower side band lters may comprise band-pass filters of conventionaldesign, the width of the pass band being determined by the selectivitycharacteristics desired for the particular receiver. For an intermediatefrequency of 455 kc., a suitable pass band width would be from 2 to 3kc.

The carrier filter 22 may also be a band-pass filter.

The width of the pass band, however, should be quite narrow and may beon the order of 2 or 3 c.p.s. or less. Therefore, it may be appreciatedthat the output from carrier filter 22 will contain only the carriercomponent of the incoming radio wave which it is desired to receiveunless, of course, interfering carriers of nearly the same -frequencyare present.

The outputs from upper side band filter 18 and carrier filter 22 arecoupled with a product detector 24. The output from carrier filter 22 isalso coupled with a product detector 26 along with the output from thelower side band filter 20. Any of a variety of product detectors ormixers commonly employed in superheterodyne receivers are suitable forproduct detectors 24 and 26. These detectors 24 and 26 employ theheterodyne principle to beat one input thereto against the other inputto produce output signals having frequencies comprising variouscombinations of the input signals.

Referring to product detector 24, one output signal therefrom will be ofa frequency equal to the frequency of the upper side band signal lessthe frequency of the carrier signal. Detection of the intelligence inthe upper side band thus occurs. Generally speaking, this differencesignal will be of audio frequency. Similarly, product detector 26produces a signal of a frequency equal to the frequency of the carriersignal less the frequency of the lower side band signal and, therefore,detection of the lower side band intelligence occurs.

The intelligence signals appearing at the outputs of product detectors24 and 26 are fed, respectively, to audio amplifiers 28 `and 30 whereinsuch signals are boosted to a suitable amplitude level for transmittalto a transducer in the form of a headset 32. Headset 32 contains a pairof earpieces 34 and 36 of conventional design. Within earpieces 34 and36 are coils 33 and 40, respectively, polarized as shown in the drawing.Those skilled in the art will appreciate that coils 38 and 40 inconventional type headsets form a part of an electromagnet whichactuates a diaphragm in accordance with the amplitude and polarity ofthe electrical signal appearing in the coil. It should be noted thatcoils 38 and 40 are grounded respectively at 42 and 44 and that thenegative side of coil 38 is connected to ground 42, while the positiveside of coil 40 is connected to ground 44.

From the foregoing it may be seen that the sound waves emanating fromearpieces 34 and 36 will be 180 degrees out-of-phase due to coils 38 and40 being oppositelyV polarized. It should be understood that therespective signals reach the earpieces in phase but that inversion ofone with respect to the other occurs when such signals are convertedinto sound.

When the listener places headset 32 in position with the earpiecescovering the ears, it is evident that the intelligence from the upperside band will be impressed exclusively upon one ear, while theintelligence from the lower side band will pass only into the other ear.Therefore, the wanted intelligence will appear to be emanating from thecenter of the listeners head in accordance with the effect previouslydescribed. Other, unwanted signals will appear to be com-ing from asource outside of the listeners head and, in the case of an interferingamplitude modulated signal of nearly the same frequency as the wantedsignal, the interference will actually seem to be moving. The movingphenomenon is caused by the fact that the de-modulating carrieremanating from carrier filter 22 and utilized by the product detectorsto demodulate the respective side bands is of the proper phase for onlythose desired side bands. Therefore, interfering side band signalswithin the pass bands of the filters 18 and Ztl will not be providedwith a proper carrier and thus will be out-of-phase with another andalso of slightly different frequencies. Hence, the phase differencebetween the interfering side band signals will be constantly changing,and it is believed that this explains the moving effect noted by thelistener.

The apparatus shown in FIG. 1, though effective, can be improved bysupplementing the carrier filter 22 with additional de-modulatingcarrier re-insertion means. It is inherent in the apparatus as shown inFIG. 1 that any interfering carrier signal within the pass band ofcarrier filter 22 will necessarily be transmitted on to the productdetectors and thus provide a proper de-rnodulating carrier for the sideband components of the undesired signal.

An improvement on the FIG. 1 scheme is shown in FIG. 2 whereinapparatuses common to both figures carry like reference numerals. Theupper and lower side bands of the incoming radio wave are separatedtherefrom in the manner as described for the system shown in FIG. l andfed to the corresponding product detectors. When a carrier istransmitted with the other components of the radio wave the carrierfilter 22 in FIG. 2 separates the carrier therefrom in like manner asfor FIG. l. The output of. the carrier filter is fed to an oscillatorstage 46 which generates an output signal that is frequency and phaselocked with the carrier signal from carrier filter 22. The carriersignal` is also conducted from the output of carrier filter 22 to anintermediate frequency phase comparator 48. Comparator 48 is coupledwith oscillator stage 46 and functions in a manner to be hereinafterdescribed. Crystal control of oscillator stage 46 is illustrated by theconnection of a piezoelectric crystal 50 to stage 46 and will bediscussed fully hereinafter.

The function of the oscillator stage 46 is to generate an electricalsignal for utilization by product detectors 24 and 26 as a de-modulatingcarrier. Such de-modulating carrier mustbe frequency and phase lockedwith the transmitted carrier signal or detection of the side bandintelligence will not occur. Minor' variations in the phase relationshipbetween the output of oscillator 46 and the transmitted carrier willeffect a degree of detection, but any substantial phase difffferencewill result in severe distortion of the intelligence 'appearing at theoutputs of the product detectors.

A simple and reliable means of lachieving the requisite phase lock isthrough the use of the crystal 50 to control the oscillator. The crystalis ground to resonate at the intermediate frequency of the receiverwhich, of course, corresponds to the frequency of the carrier afterconver sion thereof in converter 14. The output from carrier filter 22is injected into the feedback circuit of the oscillator stage and servesto trigger the oscillation of the stage and maintain such os-cillationin phase with the carrier signal.

Furthermore, the piezoelectric crystal 50 has an inherent attribute inthat the crystal is not readily susceptible to intentional jamming by afrequency modulated signal. If the oscillator stage 46 were notcrystal-controlled, other suitable means would usually have to beemployed to eliminate the tendency of the oscillator stage to follow themodulations of the frequency modulated jamming signal. The tendency tofollow or momentarily shift frequency would, of course, producedistortion at the outputs of the product detectors.

The intermediate frequency phase comparator 48 serves as a check on thephase of the signal locally generated by oscillator stage 46. Comparator48 receives the oscillator signal and the carrier signal and delivers anerror signal to the oscilla-tor st-age if the two signals are not inphase with one another. (The subject of oscillator phase control by anerror signal derived from a comparison of the phases of a pair ofelectrical signals is dealt with in references cited later in thisspecification pertaining primarily to the embodiment of FIG. 3.) Thisagain is a desirable feature if the oscillator stage were not crystalcontrolled. However, when a crystalcontrolled oscillator is utilized,the piezoelectric c-rystal inherently performs the phase comparingfunction of comparator 48 and thus renders such additional controlcircuitry unnecessary.

Cil

An amplifier 52, having its input coupled with the output of oscillatorstage 46, may be employed to boost the amplitude of the oscillatorsignal to a level sufficient for utilization lby the product detectors.A phase shift corrector 54 is interposed between the output of amplifier52 and the product detectors to correct any phase errors induced in thesignal by the circuitry of oscillator stage 46 or amplifier 52. There isfrequently a tendency for a phase shift to occur in these two stages.Therefore, the phase shift corrector 54 is adjustable to allow thereceiver to be aligned to compensate for this shift. Corrector 54 may beany of a variety of conventional phase shift networks.

The output from corrector 54 is fed to each of the product detectors 24and 26 and employed as a demo-dulating carrier in the same manner as forthe system shown in FIG. 1. Following amplification by audio amplifiers28 and 30, the intelligence signals are conducted, respectively, tospeakers S6 and 58. Alternatively, a headset such as illust-rated anddescribed for FIG. 1 may be employed instead of speakers 56 and 58. Itshould be understood that flow lines 64 and 66 from audio amplifiers 30and 28, respectively, and line 68 to oscillator 46 are not a part of theFIG. 2 embodiment.

Phase inversion of one of the intelligence signals may be accomplishedby polarizing the voice coil of one of the speakers oppositely to thatof the other speaker or, alternatively, a phase inversion stage 60 maybe interposed as shown between the output of audio amplier 30 and theinput of speaker 58. Phase inver-ter 60 may be any of a variety ofconventional phase inversion networks. It is evident that the phaseinverter may be inten posed lbetween audio amplifier 28 and speaker 56with equal effectiveness.

If speakers 56 and 58 are employed to convert the audio frequencyoutputs of the audio amplifiers into sound Waves, optimum enhancement ofAM reception is achieved when the listener is positioned between the twospeakers and equidistant therefrom. Equidistance, of course, assumesthat the output levels from the two speakers are equal in intensity.

At this juncture it should be understood that exact phase inversion ofone of the intelligence signals is not absolutely necessary for theinvention to be operable. Optimum performance is achieved when the twosignals are degrees out-of-phase with one another, but a variation ofplus or minus 5 to l5 degrees yields acceptable performance.Furthermore, some enhancement is achieved with no phase inversion bymerely directing one sound wave to one ear of the listener and the othersound wave to the listeners other ear. Without phase inversion, ofcourse, the increase in readability is considerably less than achievedwith the phase inversion technique since t-he sound phenomenon discussedearlier in this specification is not present.

In the reception of amplitude modulated, double side band signalswherein the carrier is not transmitted with the signal but fullysuppressed, the carrier filter 22 is useless in phase locking theoscillator output with the carrier signal and m-ay be omitted from thecircuitry along with comparator 48 and crystal 50, if desired. Instead,referring to FIG. 3, an audio phase and frequency sensitivediscriminator 62 is employed in cooperation with oscillator stage 46 toprovide a de-modulating carrier signal.

Flow lines 64 and 66 couple the inputs of discriminator 6-2 with therespective outputs of audio amplifiers 30 and 28. Discriminator 62compares the two intelligence signals from amplifiers 28 and 30 infrequency and in phase. The discriminator employs means whereby an errorvoltage is developed if the two intelligence signals are not in phase.This error voltage is fed from the discriminator output to theoscillator stage 46 along line 68 to control the phase of the oscillatoroutput. This provides a signal from the oscillator that is of properphase to be utilized by the product detectors in extracting theintelligence from the side bands. Thus, detection is achieved eventhough the carrier is not transmitted with the side bands.

It may be appreciated that the above detection scheme, referred to inthe art as synchronous detection, operates on the principle that likeintelligence in the two side bands of an amplitude modulated signal willbe in phase when the side bands are separately detected only if thecarrier signal component of the AM signal is employed in such detection.Therefore, when the carrier is not transmitted with the side bands, acarrier may be locally generated for de-modulation purposes by comparingthe phase of the separately detected side bands. When the detected sidebands are in phase, it is evident that a demodulating carrier of properphase is being employed. For further information on the synchronousdetection technique and the manner in which the phase of the localoscillator is controlled, see, for example, Synchronous Communications,John P. Costas, Proceedings of the I.R.E., Dec. 1956, and A SynchronousDetection Adaptor For Communications Receivers, John K. Webb CQ, June,1957.

In the light of the foregoing description of apparatus suitable forpracticing the invention, the method aspect of the invention may now bereadily understood. The method of increasing the readability of anamplitude modulated signal having `a pair of side bands comprises thefollowing steps:

First, each of the side bands is separated from the amplitude modulate-dsignal.

Secondly, each of the separated side bands is detected to obtain a pairof intelligence signals. This step is executed by providing anelectrical signal of the same frequency and phase as the carrier signalfor the side bands and combining one of the side bands with saidelectrical signal to obtain a first intelligence signal of a frequencyequal to the difference of the frequencies of the one side band and saidelectrical signal. In like manner, the other side band is combined withsaid electrical signal to obtain a second intelligence signal.

Thirdly, the first intelligence signal is converted into a first soundwave.

Fourthly, the second intelligence signal is converted into a secondsound wave substantially 180 degrees outof-phase with the first soundwave.

Fifthly, the first sound wave is directed to one ear of the listener.

Sixthly, the second sound wave is directed to the other ear of thelistener.

Conservative estimates of the results obtained by this method duringactual tests are as follows With random atmospherics interfering withthe wanted signal the enhancement of the wanted signal was yat least 2to 4 decibels over conventional amplitude modulation receptiontechniques where both receivers had equivalent band widths. With heavyheterodyning, side band interference, and jamming, the apparentenhancement was at least l() to l5 decibels. It is apparent, therefore,that many signals will be rendered readable over interference whichwould otherwise be impossible to copy.

The method of the present invention can also be applied to data ICW andtone RTTY which is transmitted by normal AM or double side bandsuppressed carrier. ln these cases the upper side band and the invertedlower side band (or vice versa) are compared and only that signal whichexactly cancels the other signal is passed on to the subsequent dataconverter.

Having thus described the invention, what is claimed as new and desiredto be` secured by Letters Patent is:

1. Apparatus for increasing the readability of an amplitude modulatedsignal having at least `a pair of side band signals as componentsthereof, said apparatus comprising:

first filter means responsive to said amplitude modulated signal 4andadapted to have the same applied thereto for attenuating the componentsof said arnplitude modulated signal except one of said side bandsignals;

second filter means responsive to said amplitude modulated signal andadapted to have the same applied thereto for attenuating the componentsof said amplitude modulated signal except the other side band signal;

means for providing a carrier signal for said side band signals; firstelectrical means coupled with said first filter means and said carrierproviding means for subtracting the frequency of the lower of said oneside band and carrier signals from the frequency of the higher thereofto produce a first intelligence signal;

second electrical means coupled with said second filter means and saidcarrier providingmeans for subtracting the frequency of the lower ofsaid other side band and carrier signals from the frequency of thehigher thereof to produce a second intelligence signal, said lcarrierproviding means comprising an oscillator stage for locally generatingsaid carrier signal and discriminator means coupled vwith said stage andresponsive to said first and second intelligence signals for controllingthe phase of said generated carrier signal t-o maintain said firstintelligence signal in phase with said second intelligence signal;

transducer means coupled with said first electrical means and responsiveto said first intelligence signal for converting the latter into a firstsound wave; and

phase reversal means coupled with said second electrical means andresponsive to said second intelligence signal for converting the latterinto a second sound wave substantially degrees out-of-phase with saidfirst sound Wave.

2. Apparatus for increasing the readability of an arnplitude modulatedsignal having a carrier signal and a pair of side band signals ascomponents thereof, said apparatus comprising:

first filter means responsive to said amplitude modulated signal andadapted to have the same applied thereto for attenuating the componentsof said amplitude modulated signal except one of said side band signals;

second filter means responsive to said amplitude modulated signal andadapted to have the same applied thereto for attenuating the componentsof said amplitude modulated signal except the other side band signal;

carrier reinsertion means including third filter means responsive tosaid amplitude modulated signal and adapted to have the same appliedthereto for attenuating the components of said amplitude modulatedsignal except said carrier signal, an oscillator stage, and meansoperably coupling said stage with said third filter means for phaselocking the oscillation of the stage with said carrier signal;

first electrical means coupled with said first filter means `and theoutput of said stage for subtracting the frequency of the lower of saidone side band and carrier signals from the frequency of the higherthereof to produce a first intelligence signal; second electrical meanscoupled with said second filter means and the output of said stage forsubtracting the frequency of the lower of said other side band andcarrier signals from the frequency of the higher thereof to produce asecond intelligence signal; transducer means coupled with said firstelectrical means and responsive to said first intelligence signal forconverting the latter intot arfirst sound wave; and

phase reversal means coupled with said second electri- References Citedby the Examiner glnie aVPOgn CHH UNITED STATES PATENTS said lrst soundWave. 5 OTHER REFERENCES 3. The invention of claim 2, said oscillatorstage iu- Snape E A IH, To Phase Or Not To Phaser, In cluding means formaintaining the Ioscillation frequency Audio pp 5'2 g8 ,an'd 59 of thestage equal to the frequency of said carrier signal.

4. The invention `o claim 2, said stage comprising a KATHLEEN H CLAFFYPfl-mary Exam-nm.'

crystal controlled oscillator `stage having a frequency of 10oscillation equal to the frequency of said carrier signal. R- LINNASSTST Exammef'

1. APPARATUS FOR INCREASING THE READABILITY OF AN AMPLITUDE MODULATEDSIGNAL HAVING AT LEAST A PAIR OF SIDE BAND SIGNALS AS COMPONENTSTHEREOF, SAID APPARATUS COMPRISING: FIRST FILTER MEANS RESPONSIVE TOSAID AMPLITUDE MODULATED SIGNAL AND ADAPTED TO HAVE THE SAME APPLIEDTHERETO FOR ATTENUATING THE COMPONENTS OF SAID AMPLITUDE MODULATEDSIGNAL EXCEPT ONE OF SAID SIDE BAND SIGNALS; SECOND FILTER MEANSRESPONSIVE TO SAID AMPLITUDE MODULATED SIGNAL AND ADAPTED TO HAVE THESAME APPLIED THERETO FOR ATTENUATING THE COMPONENTS OF SAID AMPLITUDEMODULATED SIGNAL EXCEPT THE OTHER SIDE BAND SIGNAL; MEANS FOR PROVIDINGA CARRIER SIGNAL FOR SAID SIDE BAND SIGNALS; FIRST ELECTRICAL MEANSCOUPLED WITH SAID FIRST FILTER MEANS AND SAID CARRIER PROVIDING MEANSFOR SUBSTRACTING THE FREQUENCY OF THE LOWER OF SAID ONE SIDE BAND ANDCARRIER SIGNALS FROM THE FREQUENCY OF THE HIGHER THEREOF TO PRODUCE AFIRST INTELLIGENCE SIGNAL; SECOND ELECTRICAL MEANS COUPLED WITH SAIDSECOND FILTER MEANS AND SAID CARRIER PROVIDING MEANS FOR SUBTRACTING THEFREQUENCY OF THE LOWER OF SAID OTHER SIDE BAND AND CARRIER SIGNALS FROMTHE FREQUENCY OF THE HIGHER THEREOF TO PRODUCE A SECOND INTELLIGENCESIGNAL, SAID CARRIER PROVIDING MEANS COMPRISING AN OSCILLATOR STAGE FORLOCALLY GENERATING SAID CARRIER SIGNAL AND DISCRIMINATOR MEANS COUPLEDWITH SAID STAGE AND RESPONSIVE TO SAID FIRST AND SECOND INTELLIGENCESIGNALS FOR CONTROLLING THE PHASE OF SAID GENERATED CARRIER SIGNAL TOMAINTAIN SAID FIRST INTELLIGENCE SIGNAL IN PHASE WITH SAID SECONDINTELLIGENCE SIGNAL; TRANSDUCER MEANS COUPLED WITH SAID FIRST ELECTRICALMEANS AND RESPONSIVE TO SAID FIRST INTELLIGENCE SIGNAL FOR CONVERTINGTHE LATTER INTO A FIRST SOUND WAVE; AND PHASE REVERSAL MEANS COUPLEDWITH SAID SECOND ELECTRICAL MEANS AND RESPONSIVE TO SAID SECONDINTELLIGENCE SIGNAL FOR CONVERTING THE LATTER INTO A SECOND SOUND WAVESUBSTANTIALLY 180 DEGREES OUT-OF-PHASE WITH SAID FIRST SOUND WAVE.